Efficient population transfer

The sequential STIRAP considered by Kurkal and Rice was designed to generate (0,0,0) HNC from (0,0,0) HCN. Suppose that each of the STIRAP processes is restricted to use fields that are too weak to generate efficient population transfer to the desired levels. We now examine how STIRAP -I- CDF control can assist a field intensity-limited STIRAP process to increase the desired population transfer and thereby the HCN CNH isomerization yield. In the first step of the sequential STIRAP, the (0,0,0), (2,0,1), and (5,0,1) states of HCN are chosen as the initial. 

We conclude that developing a protocol for efficient population transfer between a subset of states in a physical system requires a careful examination of the influence of background states. An analysis that is based only on the properties of a small subset of states may not be robust when those states are embedded in a dense manifold of other states. 

In contrast to weak-held (perturbative) quantum control schemes where the population of the initial state is approximately constant during the interaction with the external light held, the strong-held (nonperturbative) regime is characterized by efficient population transfer. Adiabatic strong-held techniques such as rapid... 

The realization of SPODS via PL, that is, impulsive excitation and discrete temporal phase variations, benefits from high peak intensities inherent to short laser pulses. In view of multistate excitation scenarios, this enables highly efficient population transfer to the target states (see Section 6.3.3). Furthermore, PL can be implemented on very short timescales, which is desirable in order to outperform rapid intramolecular energy redistribution or decoherence processes. On the other hand, since PL is an impulsive scenario, it is sensitive to pulse parameters such as detuning and intensity [44]. A robust realization of SPODS is achieved by the use of adiabatic techniques. The underlying physical mechanism will be discussed next. 

Figure 6.9 Generic five-state system for ultrafast efficient switching. The resonant two-state system of Figure 6.6 is extended by three target states for selective excitation. While the intermediate target state 4) is in exact two-photon resonance with the laser pulse, both outer target states 3) and 5) lie well outside the bandwidth of the two-photon spectrum. Therefore, these states are energetically inaccessible under weak-field excitation. Intense femtosecond laser pulses, however, utilize the resonant AC Stark effect to modify the energy landscape. As a result, new excitation pathways open up, enabling efficient population transfer to the outer target states as well.

For the positively chirped (PC) pulses and small detuning, relaxation does not hinder a coherent population transfer. Moreover, under these conditions the relaxation favors more efficient population transfer with respect to the system with frozen nuclear motion. 

A ballistic wavepacket motion is incompatible with a tunneling process of the proton from the enol to the keto site. The transition probability of a single attempt is much smaller than 1 and many tunnel events are necessary for an efficient population transfer leading to a gradual population rise in the product state. However, if the proton itself would move from the enol to the keto site via a barrierless path, the ESIPT would take less than 10 fs because of the small proton mass [18]. This is a first indication that slower motions of the molecular skeleton are the speed determining factors and that the proton mode is not the relevant reaction coordinate [27]. 

B. Glushko, B. Kryzhanovsky Radiative and collisional damping effects on efficient population transfer in a three-level system driven by two delayed laser pulses, Phys. Rev. A 46 (1992) 2823. 

The laser pulse parameters used in our ARPA simulations are optimized [17] for a 100 p,K thermal ensemble of Rb atoms. The pump and dump overlap time is chosen to match the average coherence time of a colliding atom pair at 100 p,K, and the spectral widths of the pulses are chosen as the energy spread of the thermal ensemble. The laser pulse durations define their intensities because the area under the pulse envelope must be sufficiently large to ensure adiabaticity and efficient population transfer. (Note, the pulse intensities in this work are higher than those in Ref. [17], our main interest being the study of interference effects rather than searching for the lowest acceptable values of laser power.)... 

The states arising from the. .. Ip Ss configuration of Ne have very nearly the same energy as that of the 2 Sq state of He so that collisional energy transfer results in efficient population of these Ne states. Similarly, the states arising from the. .. configuration... 

The laser parameters should be chosen so that a and p can make the nonadiabatic transition probability V as close to unity as possible. Figure 34 depicts the probability P 2 as a function of a and p. There are some areas in which the probabilty is larger than 0.9, such as those around (ot= 1.20, p = 0.85), (ot = 0.53, p = 2.40), (a = 0.38, p = 3.31), and so on. Due to the coordinate dependence of the potential difference A(x) and the transition dipole moment p(x), it is generally impossible to achieve perfect excitation of the wave packet by a single quadratically chirped laser pulse. However, a very high efficiency of the population transfer is possible without significant deformation of the shape of the wave packet, if we locate the wave packet parameters inside one of these islands. The biggest, thus the most useful island, is around ot = 1.20, p = 0.85. The transition probability P 2 is > 0.9, if... 

Because the Stokes pulse precedes but overlaps the pump pulse, initially Up and all population initially in field-free state 11) coincides with flo(0)- At the final time, ilp Q5 so all of the population in flo(0) projects onto the target state 6). Note that flo(0) has no projeetion on the intermediate field-free state 5 ). The Rabi frequencies of the Stokes and pump pulses that are required for efficient STIRAP-generated population transfer satisfy the condition [66]... 

So as to study the stability of the efficiency of the population transfer driven by a variable CDF, we represent the total driving field in the form... 

Figure 3.9 Comparison of the efficiency of the STIRAP + CDF and the CDF alone controls with FWHM=215ps and T -Tg = FWHM/(2 /ln 2) for various A in the population transfer 11) -> 6). (From Ref. 67).

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